Pulse oximetry system and techniques for deriving cardiac and breathing parameters from extra-thoracic blood flow measurements

ABSTRACT

Medical devices and techniques derive breath rate, breath distention, and pulse distention measurements of a subject from a pulse oximeter system coupled to a subject. These parameters, together with the conventional physiologic parameters obtained from a pulse oximeter system, can be used to assist in controlling the ventilation levels and the anesthesia levels of the subject. The development has human applications and particular applications for animal research.

RELATED APPLICATIONS

The present application claims the benefit of provisional patentapplication Ser. No. 60/826,530 entitled “Medical Devices and Techniquesfor Deriving Cardiac and Breathing Parameters from Extra-thoracic BloodFlow Measurements and for Controlling Anesthesia Levels and VentilationLevels in Subjects” filed Sep. 21, 2006.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to pulse oximeter techniques for derivingcardiac and breathing parameters of a subject from extra-thoracic bloodflow measurements, in particular the invention relates to medicaldevices and techniques for deriving breath rate, breath distention, andpulse distention measurements of a subject from a pulse oximeter systemcoupled to the subject.

2. Background Information

As background, one type of non-invasive physiologic sensor is a pulsemonitor, also called a photoplethysmograph, which typically incorporatesan incandescent lamp or light emitting diode (LED) to trans-illuminatean area of the subject, e.g. an appendage, which contains a sufficientamount of blood. FIG. 1 schematically illustrates thephotoplethysmographic phenomenon. The light from the light source 10disperses throughout the appendage, which is broken down in FIG. 1 intonon-arterial blood components 12, non-pulsatile arterial blood 14 andpulsatile blood 16, and a light detector 18, such as a photodiode, isplaced on the opposite side of the appendage to record the receivedlight. Due to the absorption of light by the appendage's tissues andblood 12, 14 and 16, the intensity of light received by the photodiode18 is less than the intensity of light transmitted by the LED 10. Of thelight that is received, only a small portion (that effected by pulsatilearterial blood 16), usually only about two percent of the lightreceived, behaves in a pulsatile fashion. The beating heart of thesubject, and the breathing of the subject as discussed below, createsthis pulsatile behavior. The “pulsatile portion light” is the signal ofinterest and is shown at 20, and effectively forms thephotoplethysmograph. The absorption described above can beconceptualized as AC and DC components. The arterial vessels change insize with the beating of the heart and the breathing of the patient. Thechange in arterial vessel size causes the path length of light to changefrom d_(min) to d_(max). This change in path length produces the ACsignal 20 on the photo-detector, I_(L) to I_(H). The AC Signal 20 is,therefore, also known as the photo-plethysmograph.

The absorption of certain wavelengths of light is also related to oxygensaturation levels of the hemoglobin in the blood transfusing theilluminated tissue. In a similar manner to the pulse monitoring, thevariation in the light absorption caused by the change in oxygensaturation of the blood allows for the sensors to provide a directmeasurement of arterial oxygen saturation, and when used in this contextthe devices are known as oximeters. The use of such sensors for bothpulse monitoring and oxygenation monitoring is known and in such typicaluses the devices are often referred to as pulse oximeters. These devicesare well known for use in humans and large mammals and are described inU.S. Pat. Nos. 4,621,643; 4,700,708 and 4,830,014 which are incorporatedherein by reference.

Current commercial pulse oximeters do not have the capability to measurebreath rate or other breathing-related parameters other than bloodoxygenation. An indirect (i.e. not positioned within the airway orairstream of the subject), non-invasive method for measuring breath rateis with impedance belts.

It is well established that it is critical to properly controlanesthesia levels of a patient, or subject. In dealing with non-humansubjects in animal research applications, having specializedanesthesiologists or specialized equipment is simply not an option forresearchers. The use of breath-related parameters and heart-relatedparameters from easily applied non-invasive sensors to automate orassist in the control of proper anesthesia levels of a subject would beof great assistance. In a similar manner, simple, easy feedback forproper ventilation control from non-invasive, easily applied sensors inanimal research applications would be very beneficial. Obviously, suchadvances would not be limited to animal research as non-invasivephysiologic measurements can be very useful for human applications aswell.

It is an object of the present invention to minimize the drawbacks ofthe existing systems and to provide medical devices and techniques forderiving cardiac and breathing parameters of a subject fromextra-thoracic blood flow measurements and for controlling theventilation levels and the anesthesia levels of a subject based uponsaid measurements.

SUMMARY OF THE INVENTION

It is noted that, as used in this specification and the appended claims,the singular forms “a,” “an,” and “the” include plural referents unlessexpressly and unequivocally limited to one referent. For the purposes ofthis specification, unless otherwise indicated, all numbers expressingany parameters used in the specification and claims are to be understoodas being modified in all instances by the term “about.” All numericalranges herein include all numerical values and ranges of all numericalvalues within the recited numerical ranges.

The various embodiments and examples of the present invention aspresented herein are understood to be illustrative of the presentinvention and not restrictive thereof and are non-limiting with respectto the scope of the invention.

At least some of the above stated objects are achieved with a method ofutilizing a conventional pulse oximeter signal to derive breath rate. Asunderstood by those of ordinary skill in the art a pulse oximeter isapplied to the subject with a simple externally applied clip. Thus, inaddition to getting oxygen saturation and heart rate from a pulseoximeter, the pulse oximeter according to the present invention canderive breath rate.

A measurement of breath rate from a pulse oximeter was first madecommercially available in December 2005 by the assignee of the presentapplication, Starr Life Sciences and is provided in the MouseOx™ devicethat was particularly designed for use with small mammals, namely ratsand mice. In this device, the breath rate is obtained by screening outthe frequency band around the heart rate point on the Fast FourierTransform (known as FFT) that is used to identify the heart rate. Thenext largest amplitude to the left (or lower frequency) of the heartrate rejection band on the FFT was considered to be the breath rate. Thevalue is then simply averaged then displayed on the screen to the user.Although useful there is room to greatly improve this calculationmethodology to assure consistent accurate results.

The currently preferred breath rate algorithm works, in a general sense,by selectively filtering the heart rate from the light signal, thenreconstructing the breath signal in the absence of the heart rate.

In addition to calculating a numerical breath rate using only pulseoximeter inputs, the present invention also provides a display of thebreath rate signal, which is presented as the Breath Pleth (short forplethysmograph). The signal is derived from the inverse FFT of thecalculations described above. It is preferred if the Breath Pleth signalis illustrated congruently with the heart signal. The reason fordisplaying the signals congruently is to avoid confusion over whichsignal represents breathing, and to illustrate the underlying breathingwaveform in conjunction with the heart signal. The utility of this plotis to provide a visual sense of the relative breath rate as comparedwith heart rate, and to allow the user to see that the heart rate andbreathing signals are superimposed on the raw infrared light signal. Onecan also deduce a relative magnitude between the signal strength due tothe heart pulse, and that due to breathing.

In addition to the breath rate calculation from the pulse oximetermeasurements, the present system provides additional breath andheart-related parameters other than the conventional heart rate andblood oxygenation. Namely the present system can calculate and displayarterial distention measurements. The distention measurements arecalculated using Beer's Law mathematics, in conjunction with the currentcalculation of oxygen saturation. There are two types of distention. Thefirst, called pulse distention, is a measurement of the arterialdistention which results from the blood pulse to the periphery due tocardiac pumping. The second, called breath distention, is a measurementof the arterial distention which results from the pulse of blood to theperiphery due to breathing effort and its effect on thoracic arterialvasculature.

As will be described below, these measurements can be particularlyuseful to assist in control of anesthesia levels and ventilationcontrols. The user can employ the measured distention to assess thestrength and quality of signals for making all sensor measurements.Further, the distention measurements, such as pulse distention, can beused to assess changes in peripheral blood flow either by changes incardiac output or by changes in vaso-active response. The breathdistention measurements may be used to assess intrapleural orintrathoracic pressure. The breath distention measurements may be usedto assess work of breathing of the subject. The distention measurementsmay have many other clinical and research applications.

A measurement of pulse distention from a pulse oximeter was first madecommercially available in December 2005 by the assignee of the presentapplication, Starr Life Sciences and provided in the MouseOX™ devicethat was particularly designed for use with small mammals, namely ratsand mice. Breath distention measurements from pulse oximetry systemshave not been previously commercially available.

Preferably the measured pulse and breath distention measurements aredisplayed together on the same plot to the user. The utility of showingthem together is that pulse distention can be used as a sort ofbaseline. The relative level of breath distention can then be used as anindicator of work of breathing. Since both are derived from changes inperipheral blood flow due to their respective mechanisms, if they bothhave the same magnitude, then both are affecting the peripheral bloodflow by the same amount. In the general case, one would expect the bloodpulse to provide a greater peripheral blood flow than would breathingeffort. However, if breath distention is greater than pulse distention,the subject is likely laboring hard to breathe, a condition that oftenresults form too much anesthesia.

The applicants have found that an increase in the breath distentionmeasurement coupled with a decrease in the blood oxygenation and a dropin one or both of the breath rate and the heart rate is indicative ofthe subject moving to a higher or deeper anesthesia level. Thetechnician can observe such trends and compensate accordingly.Additionally, appropriate thresholds can be incorporated into the systemto provide alarms and/or automated anesthesia controls to automate theprocess. These parameters are also indicative of the subject moving toan undesired lower anesthesia level and the present system provides thisinformation to the user as well. Alarms and/or automated anesthesiacontrols can be incorporated in response to detected significantmovements in the anesthesia levels.

The applicants have found that “gasping” of the subject can be detectedand is also typically indicative of a too high or deep of a level ofanesthesia, and this can be used to control the anesthesia levels bygiving appropriate feedback to the user. Further, applicants have foundthat, at least in mice, a breath distention measurement that is roughlyequal to or less than the pulse distention is indicative of properanesthesia levels and proper ventilation settings. An increase in thebreath distention measurement relative to the pulse distentionmeasurement can be used as an indicator for possible improperventilation settings. Note that it is not necessary to compare pulse andbreath distention measurements simultaneously to draw such conclusions,but viewing them together can show that the effect is only on one or theother distention measurement, and not both. The relative ratio betweenthe breath distention and the pulse distention measurements and theblood oxygenation measurement can be used to indicate proper ventilatorsetting with thresholds being set to automate the system (i.e.measurements beyond the set thresholds will activate “alarms” and/orautomate adjustments to the ventilator).

In one non-limiting aspect of the present invention a non-invasive pulseoximetry system comprises a light source emitting at least two lightsignals having distinct wavelengths directed at an appendage of asubject; and a light receiver mounted adjacent to said appendage andwhich receives said light signals; wherein the pulse oximetry systemderives at least a heart rate value, a blood oxygenation value and abreath distention value from said received light signals. The system mayderive a breath rate that is calculated by filtering the receivedsignals to remove the heart rate component thereof, then reconstructinga breath signal in the absence of the heart rate components and whereinthe breath rate is calculated using the breath signal. The breathcomponents of the received signals may be filtered prior toreconstructing the breath signal. The system may calculate arterialpulse distention measurements.

In one non-limiting aspect of the present invention a non-invasive pulseoximetry system comprises a light source adapted to be attached to anexternal appendage of a subject and configured to emit at least twodistinct wavelengths of light directed at the appendage; and a receiveradapted to be attached to the external appendage of a subject andconfigured to receive the light from the light source that has beendirected at the appendage and generating received signals there from,wherein the pulse oximetry system derives a breath rate of the subjectfrom the received signals, wherein the breath rate is calculated byfiltering the received signals to remove heart rate components thereof,then reconstructing a breath signal in the absence of the heart ratecomponents and wherein the breath rate is calculated using thereconstructed breath signal.

In one non-limiting embodiment a non-invasive pulse oximetry system fora small mammal comprises a light source adapted to be attached to anexternal appendage of a small mammal and configured to emit at least twodistinct wavelengths of light directed at the appendage; and a receiveradapted to be attached to the external appendage of a subject smallmammal and configured to receive the light from the light source thathas been directed at the appendage and generating received signals therefrom, wherein the pulse oximetry system derives a breath rate of thesubject from the received signals. The system may be mounted to the tailof a subject animal.

In one non-limiting embodiment of the present invention a non-invasivepulse oximetry system comprises a light source adapted to be attached toan external appendage of a subject and configured to emit at least twodistinct wavelengths of light directed at the appendage; and a receiveradapted to be attached to the external appendage of a subject andconfigured to receive the light from the light source that has beendirected at the appendage and generating received signals there from,wherein the pulse oximetry system derives at least one physiologicparameter of the subject from the received signals by performing an FFTon at least one time domain signal to generate a frequency domainrepresentation of the at least one time domain signal, filtering thetransformed time domain signal in the frequency domain, performing aninverse FFT on the filtered FFT signal to form a filtered time domainsignal, and calculating the at least one physiologic parameter bymeasuring the filtered time domain signal.

These and other advantages of the present invention will be clarified inthe brief description of the preferred embodiment taken together withthe drawings in which like reference numerals represent like elementsthroughout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates the photoplethysmographic phenomenon asgenerally known in the art;

FIG. 2 is a schematic view of a pulse oximeter system according to oneaspect of the present invention in which the pulse oximetry system isdesigned for small mammals such as mice and rats;

FIGS. 3-4 are perspective views of the pulse oximeter of FIG. 2 coupledto a subject, namely a mouse;

FIG. 5 is a graph of a representative signal of the raw-time domainsignal from the pulse oximeter of FIGS. 2-4;

FIG. 6 is a graph of an FFT of the signal of FIG. 5;

FIG. 7 is a graph of the FFT of FIG. 6 with the heart components thereoffiltered out in accordance with the present invention;

FIG. 8 is a graph of the FFT of FIG. 7 with the breath component filterapplied in accordance with one aspect of the present invention;

FIG. 9 is a graph of a calculated breath signal from the FFT of FIG. 8;

FIG. 10 is a representative sample of a combined display of thecalculated breath signal and combined heart signal from the systemaccording to the present invention;

FIG. 11 is a representative example of a display of the pulse distentionmeasurement and breath distention measurement in accordance with thesystem of the present invention;

FIG. 12-14 are representative screen shots of the displayed parametersfor properly anesthetized, under anesthetized and over anesthetizedsubjects, respectively.

FIG. 15 is a representative sample of a combined display of thecalculated breath signal and combined heart signal from the systemaccording to the present invention illustrating a gasping subject;

FIG. 16 is the raw-time domain signal from the pulse oximeter of FIGS.2-4, associated with the gasping subject of FIG. 15;

FIG. 17 is raw-time domain signal from the pulse oximeter of FIGS. 2-4,associated with normal response for comparison with the gasping subjectof FIG. 16.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 2-4 illustrate a pulse oximeter system 100 according to one aspectof the present invention in which the pulse oximetry system 100 isdesigned for subjects 110, namely small mammals such as mice and rats.The system 100 includes a conventional light source 120, conventionallya pair of LED light sources one being infrared and the other being red.The system 100 includes a conventional receiver 130, typically aphoto-diode. The light source 120 and receiver 130 are adapted to beattached to an external appendage of a subject 110, and may be securedto a spring-biased clip 140 or other coupling device such as tapeadhesives or the like. FIGS. 2-4 illustrate a specialized clip fromStarr Life Sciences that is configured to securely attach to the tail ofa subject 110, but any conventional clip could be used. The system 100is also coupled to a controller and display unit 150, which can be a laptop computer. The use of a lap top computer as opposed to a dedicatedcontroller and display system 150 has advantages in the researchenvironment.

The system 100 will calculate the heart rate and blood oxygenation forthe subject 110 as generally known in the art of photoplethysmograghy,and does not form the basis of the present invention. Where the subject110 is a rodent, such as a mouse or rat, care must be taken to obtainaccurate heart rate and oxygenation readings with conventional pulseoximeters due to the physiology of the subjects. Starr Life Scienceshave developed pulse oximeters that accommodate rodents under theMouseOx™ brand name. For the purpose of this application the calculationof the pulse rate, pulse signal, and blood oxygenation will beconsidered as conventional.

A first measurement of breath rate from a pulse oximeter was first madecommercially available in December 2005 by the assignee of the presentapplication, Star Life Science and provided in the MouseOx™ device thatwas particularly designed for use with small mammals, namely rats andmice. In this first method, an FFT, represented in FIG. 6, is createdfor a received signal from the infrared LED in the time-domain,represented in FIG. 5. The breath rate is obtained by screening out thefrequency band around the heart rate point on the FFT, represented inFIG. 6, that is used to identify the heart rate. The heart rate iseffectively the largest peak shown in the FFT. The peak to the right ofthe FFT represents a first harmonic of the heart rate. The peak to theleft of the heart rate on the FFT represents the measured breath rate.

The frequency band around the heart rate peak is preferably proportional(through a linear function or other relationship) to the heart rateitself, whereby the band will become larger for larger heart rates. Thisexpanding filter band will accommodate the spreading of the illustratedpeak that is expected at the higher measured heart rates. The filteringof the band is required to be sure that the peak measuring algorithmdoes not merely select the cut-off point of the heart rate peak as acalculated, but erroneous, breath rate. The next largest amplitude tothe left (or lower frequency) of the heart rate rejection band on theFFT is considered to be the breath rate in this original methodology.The breath rate value is then simply averaged then displayed on thescreen to the user. Although useful there is room to greatly improvethis breath rate calculation methodology to assure consistent accurateresults.

A preferred breath rate algorithm works, in a general sense, byselectively filtering the heart rate from the infrared light signal,then reconstructing the breath signal in the absence of the heart rate.

Specifically, the algorithm for obtaining a breath signal is as follows:Similar to the first method, an FFT, represented in FIG. 6, is createdfor a received signal from the infrared LED in the time-domain,represented in FIG. 5. In FIG. 6, the large spike is the heart rate, thesmall spike to the right is a harmonic of the heart rate, and the smallspike to the left is the breathing signal. Consequently, the frequencylocated at the highest amplitude point in the FFT is considered torepresent the heart rate. Because data used in the FFT occur over a spanof time, the heart rate can naturally drift during this period, causingthe frequency content at the peak amplitude point on the FFT to bespread over a few surrounding frequency bins.

The preferred breathing rate calculation method is to first remove allheart rate-derived frequency content from the FFT signal, called heartcomponents of the signal. The algorithm chooses a lower threshold to thelower end of the peak heart rate frequency that defines the point abovewhich all content will be removed. This can be done by digitalfiltering, but also by simply zeroing all frequency bins to the right ofthe lower threshold cutoff of the heart rate spike all the way to theend of the FFT. The lower threshold is chosen by an algorithm that isbased on the mean value of the heart rate. The lower threshold isfarther from the heart rate (i.e., the heart rate band of the FFT islarger) at high heart rates, and closer to the heart rate peak at lowheart rates. It is desired to have the heart rate band to be as narrowas possible, in order to retain the largest possible breathing frequencyspectrum. FIG. 7 illustrates a sample of the heart components removedfrom the FFT in the breathing rate calculation method of the presentinvention.

A peak detection algorithm is then used to identify the largest peakremaining in the FFT. The largest remaining peak is believed to beindicative of the breathing rate, however the preferred method performsa “breathing component filtering” on this remaining data.

This filtering application operates as follows: the initial breathingpeak is compared with the rest of the remaining bandwidth. If the chosenbreathing peak is “significantly stronger” than the others, then thebreathing filtering is effectively a zeroing of all frequency bins aminimum number of bins to the right of this peak. The minimum number ofbins has been found to be 0-3 and most preferably 2. This result isshown in FIG. 8. Significantly stronger means that the value of the“breathing peak” is greater than a predetermined factor of ALL of theother values with the heart components removed. 1.5 has been usedeffectively as the predetermined factor for calculating the relativestrength of the breathing peak. If the chosen peak is only “moderatelystronger” than the remaining peaks, then the next highest peak to theleft of the strongest breathing peak is selected, and then all points onthe FFT a minimum number to the right of this new peak are zeroed outresulting, effectively, in a graph as shown in FIG. 8 (except theBreathing filter has “pushed” the remaining breathing signal componentsto the lower frequencies). “Moderately stronger” means that less than acritical number, such as ½, of all the remaining points (but at leastsome of the remaining points) fail to satisfy the significantly strongerrequirement discussed above. Finally, if the original chosen breathingpeak is only “weakly stronger” than the remaining peaks, then thebreathing component filter will identify the next two highest peaks tothe left of the strongest peak, choose the one further to the left, thenzero all points a minimum number of bins to the right of this new peak.Weakly stronger will mean that more than a critical number, such as ½,of all the remaining points fail to satisfy the significantly strongerrequirement discussed above.

The next step in the process is to conduct an inverse FFT on theremaining frequency content as shown in FIG. 8. The breathing frequencyis then contained in this time-domain signal, as represented in FIG. 9.A peak and valley detection algorithm, graphically shown in FIG. 9, isthen used to find the breath rate. This breathing rate value iscalculated from a number of separate, serial FFT-inverse FFT pairs, andis displayed on the screen to the user.

In addition to calculating a numerical breath rate, the presentinvention also provides a display of the breath rate signal, which iscalled the Breath Pleth (short for plethysmograph). The signal isderived from the inverse FFT calculations described above. An example ofthe Breath Pleth screen is given in FIG. 10. In this picture, there aretwo plots. The underlying wave-shape represents the breathing waveformor signal. As it is depicted here, the actual plot of the breathingsignal would be the envelope of that wave shape. The reason fordisplaying it in this manner is to avoid confusion over which signalrepresents breathing, and to illustrate the underlying breathingwaveform in conjunction with the combined heart signal. This heartsignal is presented in the other line waveform (at a significantlyhigher frequency). This signal contains not only the heart rate, but allfrequency content in the received infrared light signal, and thus isreferred to in this application as the combined heart signal and alsothe raw signal. The utility of this combined plot is to provide a visualsense of the relative breath rate as compared with heart rate, and toallow the user to see that the heart rate and breathing signals aresuperimposed on the raw infrared light signal. One can also deduce arelative magnitude between the signal strength due to the heart pulse,and that due to breathing.

In addition to the breath rate calculation from the pulse oximetermeasurements, the present system 100 provides additional breath andheart-related parameters other than the conventional heart rate andblood oxygenation. Namely the present system can calculate and displayarterial distention measurements. Distention measurements are calculatedusing Beer's Law mathematics, in conjunction with the currentcalculation of oxygen saturation. There are two types of distention. Thefirst, called pulse distention, results from the blood pulse to theperiphery due to cardiac pumping. The second, called breath distention,results from the pulse of blood to the periphery due to breathing effortand its effect on thoracic arterial vasculature.

To describe the physical meaning of a distention, one must firstconsider the column of light that passes between the LED andphotodetector located on either side of the sensor clip. This light isabsorbed by all intervening tissue, but we are interested only inarterial blood. Restricting received light information to arterial bloodis done by looking for a change in light signal strength at either heartor breathing frequencies. This change literally corresponds to a changein local blood flow between the sensor heads that occurs as a result ofeither a cardiac output pulse, or a breath effort effect on the thoracicvasculature.

Next consider a cylindrical volume of arterial blood, where thecross-sectional area of the cylinder is defined by the lateraldimensions of the light column, while the height is defined by thequantity of arterial blood in the direction of the light path withinthat lateral area. Distention is then simply the change in height of thecylinder between the peak and valley of the attendant change mechanism(heart pulse or breath effort). In other words, if looking at pulsedistention, which is derived from the cardiac pulse, the distention isdue to the height of the blood flow change between systole and diastole.Likewise, the breath distention is the change in height derived from theendpoints of the breathing effort from inhale to exhale. Both distentionmeasurements are given in linear dimensional units (e.g. μm). Currentcommercial pulse oximeters, other than the current MouseOx™ product ofStarr Life Sciences, do not provide the user the capability to measureeither of these distention values, and there is no known alternativemethod for making either of these measurements.

Pulse distention can be used by the operator to assess the strength andquality of signals for making all sensor measurements to evaluate theoperation of the system. Further, It can be used to assess changes inperipheral blood flow either by changes in cardiac output or by changesin vaso-active response. Pulse distention is calculated from Beer's Law.It uses the light strength measured at systole and diastole in itscalculation. The algorithm is as follows: (a) All signal filtering, bothanalog and digital is removed from the received raw infrared lightsignal; (b) The peaks and valleys of the received infrared light signalare detected; (c) For every peak and valley pair, the ratio of the peakand valley magnitude is used in the Beer's Law formulation to obtainpulse distention; and a few pulse distention values are averaged, thendisplayed both numerically and graphically.

Breath distention is a new parameter for researchers to utilize. Theutility of breath distention includes that it can be used to assessintrapleural or intrathoracic pressure, and that it may be used toassess work of breathing. Further, it may be used to assess the level ofanesthesia. Breath distention is also calculated from Beer's Law. Thebreath distention is calculated from the inverse FFT signal as describedabove. A simple algorithm of its derivation is given as follows: (a)From the description of the breath rate calculation algorithm givenabove, we start with the FFT signal from which the heart rate is removedonly (FIG. 7), before additional frequency content clipping occurs withthe breathing component filtering. Starting with this FFT, all originalsignal filtering, both analog and digital is removed by compensating theFFT amplitudes at each frequency bin, based on original filtering; (b)Once the filtering has been compensated, an inverse FFT is conducted;(c) The peaks and valleys of the inverse FFT time-domain breathingsignal are identified; (d) All of the valid peaks are averaged, then allof the valid valleys are averaged; (e) From the average peak and valleypair for each FFT dataset, the Beer's Law calculation is used to findthe breath distention; and (f) A few breath distention values areaveraged, then displayed both numerically and graphically.

Pulse and breath distention will be displayed together on the same plotin the Monitor Subject screen such as the display of the lap top 150,which is shown in FIG. 11. The utility of showing the distentionmeasurements together is that pulse distention can be used as a sort ofbaseline. The relative level of breath distention can then be used as anindicator of work of breathing. Since both are derived from changes inperipheral blood flow due to their respective mechanisms, if they bothhave the same magnitude, then both are affecting the peripheral bloodflow by the same amount. In the general case, one would expect the bloodpulse to provide a greater peripheral blood flow than would breathingeffort. However, if breath distention is greater than pulse distention,the animal is likely laboring hard to breathe, a condition that oftenresults form too much anesthesia.

The present system 10 effectively provides a method of controlling theanesthesia level and/or ventilator settings of a subject that isreceiving anesthesia and/or respiratory support through a ventilator.The method comprises the steps of providing the non-invasive sensorsystem 100 configured to calculate arterial pulse distentionmeasurements of the subject, and using the measured arterial pulsedistention measurements as indicators for at least one of proper andimproper levels of anesthesia or proper and improper ventilator controlsettings. This method may be clarified in a review of FIGS. 12-17.

The applicants have found that an increase in the breath distentionmeasurement coupled with a decrease in the blood oxygenation and a dropin one or both of the breath rate and the heart rate is indicative ofthe subject moving to a higher or deeper anesthesia level. Thetechnician can observe such trends and compensate accordingly.Additionally, appropriate thresholds can be incorporated into the systemto provide alarms and/or automated anesthesia controls to automate theprocess. These parameters are also indicative of the subject moving toan undesired lower anesthesia level and the present system provides thisinformation to the user as well. Alarms and/or automated anesthesiacontrols can be incorporated in response to detected significantmovements in the anesthesia levels.

FIG. 12 is a screen clipping of the display of the system 100 for asubject, specifically a mouse, that is properly anesthetized. The pulseand breath distention are basically the same, the breath rate is stableand in the proper range. FIG. 13 is a screen clipping of a subject,again a mouse, that is too lightly anesthetized. This mouse is gettingready to wake up. The breath rate is increasing and the breathdistention is much less than the pulse distention. FIG. 14 shows ascreen clipping of a subject, again a mouse, that is too heavilyanesthetized. This mouse is gasping and breathing at a very slow rate.This screen shot represents an extreme case and the breathing is verydifficult to calculate because it is so slow. This results in that thebreath distention is not updating often. However, when breath distentionis able to update, as shown it is much higher than pulse distentionproviding important feedback to the operator.

The applicants have found that “gasping” of the subject can be detectedand is also typically indicative of a too high or deep of a level ofanesthesia, and this can be used to control the anesthesia levels bygiving appropriate feedback to the user. Further, the applicants havefound that, at least in mice, a breath distention measurement that isroughly equal to or less than the pulse distention is indicative ofproper anesthesia levels and proper ventilation settings. An increase inthe breath distention measurement relative to the pulse distentionmeasurement can be used as an indicator for possible improperventilation settings. The relative ratio between the breath distentionand the pulse distention measurements and the blood oxygenationmeasurement can be used to indicate proper ventilator setting withthresholds being set to automate the system (i.e. measurements beyondthe set thresholds will activate “alarms” and/or automate adjustments tothe ventilator). For example, consider FIGS. 15 and 16 which illustratethe graphical displays indicative of a deeply anesthetized subject,again a mouse. The screen clipping of the breath pleth window display ofFIG. 15 shows a subject mouse that is too heavily anesthetized. Thismouse is gasping and breathing at a very slow rate. The user can see inthis window is that the mouse is gasping by the effect on the pulsesignal. The pulse signal displayed here actually contains both of thedistentions. The pulse distention is low for most of these heart beatsthen it will calculate high for this gasping beat. The breath distentionwill be high because it only looks at the effects cause by breathing.These parameters can be effectively used as guidance for both anesthesialevels and ventilation control.

The present system 100 is not intended to be restrictive of theinvention. For example, all of these parameters can be measured using apartially-deflated blood pressure cuff, impedance belts or an arterialline. Further, the filtering is described above using inverse FFTs, butit can be done also with traditional digital and analog filteringmethods. Additionally, reflective oximetry sensors, implanted sensors,clip-less sensor, etc could be used. Only a light source (e.g., LED) andreceiver (e.g., photodiode) are required.

Although the present invention has been described with particularityherein, the scope of the present invention is not limited to thespecific embodiment disclosed. It will be apparent to those of ordinaryskill in the art that various modifications may be made to the presentinvention without departing from the spirit and scope thereof. The scopeof the present invention is defined in the appended claims andequivalents thereto.

1. A non-invasive pulse oximetry system comprising: a controller; alight source coupled to the controller and emitting at least two lightsignals having distinct wavelengths adapted to be directed at anappendage of a subject; and a light receiver coupled to the controllerand adapted to be mounted adjacent to said appendage and which receivessaid light signals; wherein the controller of the pulse oximetry systemderives at least a heart rate value, a blood oxygenation value, a breathrate and a breath distention value from said received light signals,wherein the breath distention value is associated with the height of theblood flow change in the appendage derived from the endpoints of thebreathing effort from inhale to exhale, and wherein the system derivesthe breath rate that is calculated by filtering the received signals inthe frequency domain to remove the heart rate component thereof, thenreconstructing a time domain breath signal in the absence of the heartrate components and wherein the breath rate is calculated using thereconstructed breath signal.
 2. The non-invasive pulse oximetry systemof claim 1 wherein the breath components of the received signals arefiltered prior to reconstructing the breath signal.
 3. The non-invasivepulse oximetry system of claim 1 wherein the system calculates arterialpulse distention measurements, wherein the arterial pulse distentionmeasurements are associated with the height of the blood flow change inthe appendage derived from the endpoints of the cardiac effort betweensystole and diastole.
 4. A non-invasive pulse oximetry systemcomprising: A controller; A light source coupled to the controller andadapted to be attached to an external appendage of a subject andconfigured to emit at least two distinct wavelengths of light directedat the appendage; and A receiver coupled to the controller and adaptedto be attached to the external appendage of a subject and configured toreceive the light from the light source that has been directed at theappendage and generating received signals there from, wherein thecontroller of the pulse oximetry system derives a breath rate of thesubject from the received signals, wherein the breath rate is calculatedby transforming the received signals to the frequency domain, filteringthe received signals to remove heart rate components thereof in thefrequency domain, then reconstructing a breath signal in the time domainin the absence of the heart rate components and wherein the breath rateis calculated using the reconstructed breath signal in the time domain.5. The non-invasive pulse oximetry system of claim 4 wherein thefiltering the received signals to remove heart rate components thereofresults in a filtered signal having breath components therein andwherein the breath components of the received signals are filtered priorto reconstructing the breath signal.
 6. The non-invasive pulse oximetrysystem of claim 5 wherein the system calculates arterial pulsedistention measurements, wherein the arterial pulse distentionmeasurements are associated with the height of the blood flow change inthe appendage derived from the endpoints of the cardiac effort betweensystole and diastole.
 7. The non-invasive pulse oximetry system of claim4 wherein the system calculates arterial pulse distention measurements,wherein the arterial pulse distention measurements are associated withthe height of the blood flow change in the appendage derived from theendpoints of the cardiac effort between systole and diastole.
 8. Anon-invasive pulse oximetry system for a small mammal comprising: Acontroller; A light source coupled to the controller and adapted to beattached to an external appendage of a small mammal and configured toemit at least two distinct wavelengths of light directed at theappendage; and A receiver coupled to the controller and adapted to beattached to the external appendage of a subject small mammal andconfigured to receive the light from the light source that has beendirected at the appendage and generating received signals there from,wherein the controller of the pulse oximetry system derives a breathrate of the subject from the received signals and wherein the systemcalculates arterial pulse distention measurements, wherein the arterialpulse distention measurements are associated with the height of theblood flow change in the appendage derived from the endpoints of thecardiac effort between systole and diastole, and wherein the arterialpulse distention measurements is derived from a plurality of ratios ofpeak and valley ratios of the received signals.
 9. The non-invasivepulse oximetry system of claim 8 wherein the system is mounted to thetail of a subject animal.
 10. The non-invasive pulse oximetry system ofclaim 9 wherein the received signals includes heart rate componentsthereof and wherein the breath rate is calculated by filtering thereceived signals to remove the heart rate components thereof, thenreconstructing a breath signal in the absence of the heart ratecomponents and wherein the breath rate is calculated using the breathsignal.
 11. The non-invasive pulse oximetry system of claim 10 whereinthe breath components of the received signals are filtered prior toreconstructing the breath signal.
 12. A non-invasive pulse oximetrysystem comprising: A controller; A light source coupled to thecontroller and adapted to be attached to an external appendage of asubject and configured to emit at least two distinct wavelengths oflight directed at the appendage; and A receiver coupled to thecontroller and adapted to be attached to the external appendage of asubject and configured to receive the light from the light source thathas been directed at the appendage and generating received signals therefrom, wherein the controller of the pulse oximetry system derives atleast one physiologic parameter of the subject from the received signalsby performing an FFT on at least one time domain signal to generate afrequency domain representation of the at least one time domain signal,filtering the transformed time domain signal in the frequency domain,performing an inverse FFT on the filtered FFT signal to form a filteredtime domain signal, and calculating the at least one physiologicparameter by measuring the filtered time domain signal, wherein thesystem calculates arterial breath distention measurements of the subjectwherein the breath distention measurements are associated with theheight of the blood flow change in the appendage derived from theendpoints of the breathing effort from inhale to exhale.
 13. Thenon-invasive pulse oximetry system of claim 12 wherein the systemfurther derives a breath rate of the subject by reconstructing a timedomain breath signal in the absence of heart rate components and whereinthe breath rate is calculated using the reconstructed breath signal. 14.The non-invasive pulse oximetry system of claim 13 wherein the systemfurther derives a heart rate of the subject by reconstructing a timedomain heart signal in the absence of breath components and wherein theheart rate is calculated using the reconstructed heart signal.
 15. Thenon-invasive pulse oximetry system of claim 13 wherein the systemcalculates arterial pulse distention measurements, wherein the arterialpulse distention measurements are associated with the height of theblood flow change in the appendage derived from the endpoints of thecardiac effort between systole and diastole.
 16. The non-invasive pulseoximetry system of claim 12 wherein the system calculates arterial pulsedistention measurements, wherein the arterial pulse distentionmeasurements are associated with the height of the blood flow change inthe appendage derived from the endpoints of the cardiac effort betweensystole and diastole.